Nearly 13 years after they were jailed for life for a double murder in Wales, two men will have their appeal against their conviction heard in the court of appeal today.

The appeal is the culmination of one of Britain's longest-running campaigns over an alleged miscarriage of justice. Michael Attwooll and John Roden were found guilty in 1995 of the murder of a taxi firm owner, Gerry Stevens, and his lover, Christine Rees, who worked as a receptionist at the firm.

The couple had been attacked with a sharp instrument, which has never been found, and then shot in the head at the firm's offices in Risca, near Newport, in May 1994. The couple had been having an affair and someone had sent compromising photos of them to their respective spouses.

Attwooll was Stevens's partner in the taxi firm and the two men had known each other for 30 years. Roden was going out with Attwooll's daughter, Vicky. At the trial, it was suggested that Attwooll had been unhappy about Stevens' affair and also was concerned about the firm's accounts and business practices. He was alleged to have recruited Roden to help him carry out the murder of Stevens, and it was suggested that Rees had been killed because she was there at the time.

Both Attwooll and Roden had alibis but these were challenged in the trial. Attwooll claimed he had been at home with his wife. There was also evidence against the men from former prisoners who claimed they had implicated themselves, claims the men denied in court.

Six fellow prisoners of Attwooll made statements that he had always insisted on his innocence during his time in jail awaiting trial. Blood spots found in Attwooll's car, according to the defence, could easily have got there because Attwooll, at the request of the police, had visited the murder scene. The two men's supporters said there was nothing to suggest that Attwooll would react so violently over Stevens having an affair, as he had previously stood up for Stevens when he had been threatened over another affair.

Since the trial it has emerged that Stevens had told people that he thought someone was after him, even specifying the kind of car they were driving. Another employee, a driver, had previously been attacked and Stevens had said he believed he might have been the real target.

A campaign was launched on behalf of the men shortly after their conviction and a number of demonstrations have been held on their behalf. Bob Woffinden, author of Miscarriages of Justice, has described the case as "the most serious miscarriage of justice in Welsh history. This is the kind of case that creates a stink that never goes away."

Campaigners hailed the news that the case was finally being heard.

The case was referred back to the court of appeal by the Criminal Cases Review Commission which had previously, in 2001, decided not to refer it back.